Energy storage device

ABSTRACT

An energy storage device in which a micro-short circuit at the time of heat generation is suppressed is provided. The energy storage device includes: a positive electrode; a negative electrode containing a negative composite layer; and a separator disposed between the positive electrode and the negative electrode. The separator contains a base material layer containing a thermoplastic resin and an inorganic layer formed on the base material layer, the inorganic layer opposes to the positive electrode, the base material layer opposes to the negative electrode, and a ratio of a mass of the base material layer per unit area to a spatial volume of the negative composite layer is 0.26 or more.

TECHNICAL FIELD

The technology disclosed herein relates to an energy storage device.

BACKGROUND ART

Conventionally, as a nonaqueous electrolyte secondary battery, which isa type of energy storage device, one described in Patent Document 1 isknown. This nonaqueous electrolyte secondary battery includes a positiveelectrode, a negative electrode, and a separator disposed between thepositive electrode and the negative electrode. The negative electrodeincludes a negative composite layer formed on a surface of a negativeelectrode current collector made of a metal. In addition, the separatorincludes a base material layer containing a thermoplastic resin as amain component and an inorganic layer containing a filler as a maincomponent.

Patent Document 1 states the following. “The lithium secondary batteryof the present invention includes a positive electrode, a negativeelectrode, a nonaqueous electrolyte, and a separator enclosed in ahollow columnar battery case and has the following characteristics. Thepositive electrode includes a positive composite layer containing apositive active material, an electrically conductive auxiliary, and abinder on one or both sides of a current collector. As the positiveactive material, a lithium-containing composite oxide containing lithiumand a transition metal is used. At least part of the lithium-containingcomposite oxide is a lithium-containing composite oxide containingnickel as a transition metal, and the molar ratio of the total nickelamount relative to the total lithium amount in the entire positiveactive material is 0.05 to 1.0. The separator includes a porous membrane(I) containing a thermoplastic resin as a main component and a porouslayer (II) containing a filler having a heatproof temperature of 150° C.or higher as a main component. The side surface portion of the batterycase includes two large-width surfaces that are opposed to each otherand each have a larger width than other surfaces in side view. The sidesurface portion has formed therein a splitting groove that splits in thecase where the pressure in the battery case exceeds the threshold. Thesplitting groove is provided so as to intersect a diagonal line from thelarge-width surfaces in side view” (paragraph 0009). As a result, “alithium secondary battery having high capacity together with excellentsafety at extremely high temperatures can be provided” (paragraph 0010).

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-2013-98027

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the battery described in Patent Document 1, a separator including abase material layer containing a thermoplastic resin as a main componentand an inorganic layer containing a filler as a main component is used.Therefore, even in the case where the battery temperature excessivelyincreases in a usage form that is usually unforeseen, the separator islikely to be prevented from thermal shrinkage and bringing the positiveelectrode and the negative electrode into contact with each other. Thisis presumably because the inorganic layer functions as the backbone ofthe separator to suppress shrinkage of the separator.

The present inventor has found that in the above configuration, when thebattery temperature excessively increases in a usage form that isusually unforeseen, a micro-short circuit occurs between the positiveelectrode and the negative electrode, and the voltage of the battery mayslightly decrease. The reasons therefor are presumably as follows.

Even if the shape of the base material layer is maintained by theinorganic layer, and thermal shrinkage of the separator is therebysuppressed, when the temperature of the base material layer becomesequal to or higher than the melting point, it is concerned that the basematerial layer may melt. As a result, the molten thermoplastic resin maypenetrate into the positive electrode or the negative electrode. In theseparator, the part where the molten base material layer has penetratedinto the positive electrode or the negative electrode reduces inthickness, and, in some cases, a through-hole may be formed in the basematerial layer. As a result, it is concerned that due to thethrough-hole, a micro-short circuit may occur between the positiveelectrode and the negative electrode.

The technology disclosed herein has been accomplished against the abovebackground. An object thereof is to provide an energy storage deviceconfigured such that when the energy storage device generates heat in ausage form that is usually unforeseen, a micro-short circuit at the timeof such heat generation is suppressed.

Means for Solving the Problems

An aspect of a technology disclosed in this specification is an energystorage device including: a positive electrode; a negative electrodecontaining a negative composite layer; and a separator disposed betweenthe positive electrode and the negative electrode. The separatorcontains a base material layer containing a thermoplastic resin and aninorganic layer formed on the base material layer. The inorganic layeropposes to the positive electrode, and the base material layer opposesto the negative electrode. A ratio of a mass of the base material layerper unit area to a spatial volume of the negative composite layer is0.26 or more.

Another aspect of the technology disclosed in this specification is anenergy storage device including: a positive electrode; a negativeelectrode containing a negative composite layer; and a separatordisposed between the positive electrode and the negative electrode. Theseparator contains a base material layer containing a thermoplasticresin and an inorganic layer formed on the base material layer. Theinorganic layer opposes to the positive electrode, and the base materiallayer opposes to the negative electrode. A mass of the base materiallayer per unit area is 0.085 (g/100 cm²) or more, and a density of thenegative composite layer is 1.3 (g/cm³) or more.

Advantages of the Invention

According to the technology disclosed in this specification, amicro-short circuit can be suppressed when the energy storage devicegenerates heat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an energy storage device accordingto Embodiment 1.

FIG. 2 is a cross-sectional view showing the energy storage deviceaccording to Embodiment 1.

FIG. 3 is a schematic view showing an energy storage apparatus in whichthe energy storage device according to Embodiment 1 is provided.

FIG. 4 is an automobile with the energy storage apparatus in which theenergy storage device according to Embodiment 1.

MODE FOR CARRYING OUT THE INVENTION Summary of Embodiments

An embodiment of a technology disclosed in this specification is anenergy storage device including: a positive electrode; a negativeelectrode containing a negative composite layer; and a separatordisposed between the positive electrode and the negative electrode. Theseparator contains a base material layer containing a thermoplasticresin and an inorganic layer formed on the base material layer. Theinorganic layer opposes to the positive electrode, and the base materiallayer opposes to the negative electrode. A ratio of a mass of the basematerial layer per unit area to a spatial volume of the negativecomposite layer is 0.26 or more.

According to the configuration described above, even in the case wherethe base material layer of the separator melts, the formation of athrough-hole in the base material layer is suppressed. As a result, amicro-short circuit in the energy storage device at the time of heatgeneration can be suppressed.

When the spatial volume of the negative composite layer is too large,presumably, the molten base material layer is likely to penetrate intothe negative composite layer, and thus a through-hole is likely to beformed in the base material layer. Meanwhile, when the mass of the basematerial layer per unit area is too small, presumably, in the case wherethe base material layer melts, a through-hole is likely to be formed inthe base material layer.

Thus, in the technology disclosed herein, the ratio of the mass of thebase material layer per unit area to the spatial volume of the negativecomposite layer is specified to be 0.26 or more. As a result,presumably, even in the case where the base material layer melts, theformation of a through-hole in the base material layer can besuppressed.

As embodiments of the technology disclosed herein, the following modesare preferable.

As an embodiment of the technology disclosed in this specification, inthe above-mentioned energy storage device, it can be adopted that themass of the base material layer per unit area is 0.085 (g/100 cm²) ormore.

According to the configuration described above, by letting the mass ofthe base material layer per unit area be 0.085 (g/100 cm²) or more, evenin the case where the base material layer melts, the formation of athrough-hole in the base material layer can be further suppressed. As aresult, a micro-short circuit in the energy storage device can befurther suppressed.

As an embodiment of the technology disclosed in this specification, inthe above-mentioned energy storage device, it can be adopted that theratio of the mass of the base material layer per unit area to thespatial volume of the negative composite layer is 0.31 or more and 0.53or less.

According to the configuration described above, even in the case wherethe base material layer melts, the formation of a through-hole in thebase material layer can be further suppressed. As a result, amicro-short circuit in the energy storage device can be furthersuppressed.

An embodiment of the technology disclosed in this specification is anenergy storage device including: a positive electrode; a negativeelectrode containing a negative composite layer; and a separatordisposed between the positive electrode and the negative electrode. Theseparator contains a base material layer containing a thermoplasticresin and an inorganic layer formed on the base material layer, theinorganic layer opposes to the positive electrode, and the base materiallayer opposes to the negative electrode. A mass of the base materiallayer per unit area is 0.085 (g/100 cm²) or more, and a density of thenegative composite layer is 1.3 (g/cm³) or more.

According to the mode described above, even in the case where the basematerial layer of the separator melts, the formation of a through-holein the base material layer is suppressed. As a result, a micro-shortcircuit in the energy storage device at the time of heat generation canbe suppressed.

When the density of the negative composite layer is low, presumably, themolten base material layer is likely to penetrate into the negativecomposite layer, and thus a through-hole is likely to be formed in thebase material layer. Meanwhile, when the mass of the base material layerper unit area is too small, presumably, in the case where the basematerial layer melts, a through-hole is likely to be formed in the basematerial layer.

Thus, in the technology disclosed herein, the mass of the base materiallayer per unit area is specified to be 0.085 (g/100 cm²) or more, andthe density of the negative composite layer is specified to be 1.3(g/cm³) or more. As a result, presumably, even in the case where thebase material layer melts, the formation of a through-hole in the basematerial layer can be suppressed.

As one embodiment of the technology disclosed herein, the energy storagedevice described above may be configured such that the density of asurface part of the negative composite layer which opposes to the basematerial layer is larger than the density of a part of the negativecomposite layer adjacent to a negative electrode current collectionfoil.

According to the mode described above, the molten base material layer ismore unlikely to penetrate into the negative composite layer. As aresult, a micro-short circuit in the energy storage device can be evenmore suppressed.

As one embodiment of the technology disclosed herein, the energy storagedevice described above may be configured such that the base materiallayer contains polyethylene as the thermoplastic resin, and the contentof polyethylene is 90 mass % or more relative to the mass of the basematerial layer.

Polyethylene has a lower melting point compared with other thermoplasticresins (e.g., polypropylene), and thus is particularly effective in thecase where the technology disclosed herein is applied.

Embodiment 1

Hereinafter, Embodiment 1 will be described with reference to FIG. 1 toFIG. 4. An energy storage device according to Embodiment 1 is mounted ona vehicle main body 50 of an electric vehicle, a hybrid electricvehicle, or the like, for example, and used as a power source for anautomobile 100. The energy storage device according to Embodiment 1 is anonaqueous electrolyte secondary battery 10, more specifically alithium-ion secondary battery, including a positive electrode 18, anegative electrode 19, a separator 21, and an electrolyte (not shown)housed in a case 11. Incidentally, the nonaqueous electrolyte secondarybattery 10 is not limited to a lithium-ion secondary battery, and anarbitrary secondary battery may be selected as necessary.

(Case 11)

As shown in FIG. 1, the case 11 is made of a metal and has a flatrectangular parallelepiped shape. As the metal to form the case 11, anarbitrary metal such as iron, an iron alloy, aluminum, or an aluminumalloy may be selected as necessary.

On the upper surface of the case 11, a positive electrode terminal 16and a negative electrode terminal 17 are provided to project upwards.The positive electrode terminal 16 is electrically connected to thepositive electrode 18 in a known manner in the case 11. In addition, thenegative electrode terminal 17 is electrically connected to the negativeelectrode 19 in a known manner in the case 11.

(Energy Storage Element 20)

As shown in FIG. 2, in the case 11, an energy storage element 20composed of the positive electrode 18 and the negative electrode 19rolled with the separator 21 therebetween is housed.

(Positive Electrode 18)

A positive electrode current collection foil is in the form of a foilmade of a metal. The positive electrode current collection foilaccording to this embodiment is made of aluminum or an aluminum alloy.The thickness of the positive electrode current collection foil ispreferably 5 μm or more and 20 μm or less.

On one or both sides of the positive electrode current collection foil,a positive composite layer containing a positive active material isformed. In this embodiment, the positive composite layer is formed onboth sides of the positive electrode current collection foil. Thepositive composite layer may contain an electrically conductiveauxiliary and a binder.

As the positive active material, a publicly-known material can beappropriately used as long as the positive active material is capable ofoccluding and releasing a lithium ion. For example, as the positiveactive material, a polyanion compound selected from LiMPO₄, LiMSiO₄,LIMBO₃ or the like (M is one or more transition metal element selectedfrom Fe, Ni, Mn, Co or the like); a spinel compound such as lithiumtitanate, lithium manganese or the like, and a lithium transition metaloxide such as LiMO₂ (M is one or more transition metal element selectedfrom Fe, Ni, Mn, Co or the like) or the like can be used.

A kind of the electrically conductive auxiliary is not particularlylimited. The electrically conductive auxiliary may be metal ornon-metal. As the metal electrically conductive auxiliary, a materialincluding a metal element such as Cu, Ni or the like may be used. As anon-metal electrically conductive auxiliary, a carbon material such asgraphite, carbon black, acetylene black, ketjen black or the like may beused.

The binder is not particularly limited in kind as long as it is amaterial stable to a solvent used at manufacturing an electrode and anelectrolyte and stable to oxidation-reduction reaction duringcharge-discharge. For example, a thermoplastic resin such aspolytetrafluoroethylene(PTFE), polyvinylidene fluoride (PVDF),polyethylene, polypropylene and a polymer having a rubber elasticitysuch as ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM,styrene-butadiene rubber (SBR) and fluorine-contained rubber can be usedsolely or as a mixture of two or more thereof.

If necessary, a viscosity modifier may be contained in the positiveelectrode composite layer. As the viscosity modifier, a compound likecarboxymethyl cellulose (CMC) or the like can be appropriately selected.

(Negative Electrode 19)

A negative electrode current collection foil is in the form of a foilmade of a metal. The negative electrode current collection foilaccording to this embodiment is made of copper or a copper alloy. Thethickness of the negative electrode current collection foil ispreferably 5 μm or more and 20 μm or less.

On one or both sides of the negative electrode current collection foil,a negative composite layer containing a negative active material isformed. In this embodiment, the negative composite layer is formed onboth sides of the negative electrode current collection foil. Thenegative composite layer may contain an electrically conductiveauxiliary, a binder, and a thickener.

With respect to electrically conductive auxiliaries, binders, viscositymodifiers, and the like that may be used for the negative electrode 19,the same ones as those used for the positive electrode 18 may besuitably selected and used. Thus, the description thereof will beomitted.

As the negative active material, a carbon material, an element capableof alloying with lithium, an alloy, a metal oxide, a metal sulfide, ametal nitride or the like can be used. As an example of the carbonmaterial, hard carbon (non-graphitizable carbon), soft carbon(graphitizable carbon), graphite or the like can be used. As an exampleof the element capable of alloying with lithium, Al, Si, Zn, Ge, Cd, Sn,Pb or the like can be used. As an example of the alloy, an alloyincluding a transition metal such as Ni—Si alloy and Ti—Si alloy can beused. As an example of the metal oxide, an amorphous tin oxide such asSnB_(0.4)P_(0.6)O_(3.1) or the like, a tin-silicon oxide such as SnSiO₃or the like, silicon oxide such as SiO or the like, lithium titanatewith spinel structure such as Li_(4+x)Ti₅O₁₂ or the like can be used. Asan example of the metal sulfide, lithium sulfide such as TiS₂ or thelike, molybdenum sulfide such as MoS₂ or the like, iron sulfide such asFeS, FeS₂, Li_(x)FeS₂ or the like can be used. In the above-mentionedmaterials, graphite or hard carbon is particularly preferable.

The lower limit of the spatial volume of the negative composite layer ispreferably 0.120 (cm³/100 cm²), more preferably 0.140 (cm³/100 cm²), andstill more preferably 0.165 (cm³/100 cm²). In addition, the upper limitof the spatial volume of the negative composite layer is preferably0.380 (cm³/100 cm²), more preferably 0.360 (cm³/100 cm²), and still morepreferably 0.330 (cm³/100 cm²). Incidentally, the spatial volume of anegative composite layer means the volume of the space where thenegative composite layer is absent per unit area of the negativecomposite layer.

The lower limit of the porosity of the negative composite layer ispreferably 20%, more preferably 23%, and still more preferably 26%. Inaddition, the upper limit of the porosity of the negative compositelayer is preferably 47%, more preferably 45%, and still more preferably43%.

(Separator 21)

The base material layer of the separator 21 is not particularly limitedas long as it contains a thermoplastic resin. As the base material layerof the separator 21, a microporous polyolefin membrane, a woven fabricor a nonwoven fabric made of a synthetic resin fiber or the like can beused. As the microporous polyolefin membranes, polyethylene,polypropylene or a composite membrane thereof can be used. The syntheticresin fiber can be selected from polyacrylonitrile (PAN), polyamide(PA), polyester, polyethylene terephthalate (PET), polyolefin such aspolypropylene (PP) or polyethylene (PE), or a mixture thereof. A lowerlimit of the thickness of the separator 21 is preferably 5 μm, morepreferably 8 μm, further preferably 12 μm. An upper limit of thethickness of the separator 21 is preferably 35 μm, more preferably 25μm, further preferably 20 μm.

On one side of the base material layer of the separator 21, an inorganiclayer containing heat-resistant particles and a binder is formed. Theinorganic layer opposes to the positive electrode 18. As theheat-resistant particles, those having a weight loss of 5% or less at500° C. in the atmosphere are preferable. Among them, those having aweight loss of 5% or less at 800° C. are particularly preferable. Assuch a material, an inorganic compound can be mentioned. As inorganiccompounds, the following inorganic substances alone, or alternativelymixtures or composite compounds thereof, can be mentioned: oxideparticles such as iron oxide, SiO₂, Al₂O₃, TiO₂, BaTiO₂, ZrO, andalumina-silica composite oxide; nitride particles such as aluminumnitride and silicon nitride; poorly soluble ionic crystalline particlessuch as calcium fluoride, barium fluoride, and barium sulfate; covalentbond crystalline particles such as silicon and diamond; clay particlessuch as talc and montmorillonite; mineral resource-derived substancessuch as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine,sericite, bentonite, and mica, as well as artificial products thereofand the like. In addition, it is also possible that electricallyconductive particles, such as metal particles, oxide particles such asSnO₂ or tin-indium oxide (ITO), or carbonaceous particles such as carbonblack, are surface-treated with a material having electrical insulationproperties (e.g., a material constituting the inorganic substancedescribed above) to impart electrical insulation properties, and theresulting particles are used. Among these inorganic compounds, SiO₂,Al₂O₃, and alumina-silica composite oxide are preferable.

The binder is not limited in kind as long as it is a material stable toan electrolyte. Examples of binders include polyacrylonitrile,polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylenecopolymer, polytetrafluoroethylene, polyhexafluoropropylene,polyethylene oxide, polypropylene oxide, polyphosphazene, polysiloxane,polyvinyl acetate, polyvinyl alcohol, polymethyl acrylate, polymethylmethacrylate, polyacrylic acid, polymethacrylic acid, styrene-butadienerubber, nitrile-butadiene rubber, polystyrene, and polycarbonate. Amongthem, in terms of electrochemical stability, it is preferable that thebinder is polyacrylonitrile, polyvinylidene fluoride, styrene-butadienerubber, polyhexafluoropropylene, or polyethylene oxide. In particular,polyvinylidene fluoride or styrene-butadiene rubber is more preferable.

The thickness of the inorganic layer is preferably 3 μm or more and 10μm or less. When the thickness of the inorganic layer is 3 μm or more, amicro-short circuit between the positive electrode and the negativeelectrode can be suppressed more reliably. In addition, when thethickness of the inorganic layer is 10 μm or less, an excessive increasein the resistance of the energy storage device due to the distancebetween the positive electrode and the negative electrode can besuppressed.

The mass of the separator per unit area is preferably 0.060 (g/100 cm²)or more, and more preferably 0.085 (g/100 cm²) or more. As a result, amicro-short circuit in the energy storage device can be even moresuppressed.

(Ratio of Mass of Base Material Layer Per Unit Area to Spatial Volume ofNegative Composite Layer)

The ratio of the mass of the base material layer per unit area to thespatial volume of the negative composite layer is 0.26 or more.

When the spatial volume of the negative composite layer is large, in thecase where the base material layer of the separator melts, there is atendency that the molten base material layer is likely to penetrate intothe negative composite layer. As a result, it is concerned that athrough-hole is formed in the base material layer of the separator, and,due to the through-hole, a micro-short circuit may occur between thepositive electrode 18 and the negative electrode 19.

In addition, when the mass of the base material layer per unit area issmall, in the case where the base material layer of the separator melts,a through-hole is likely to be formed in the base material layer. It isconcerned that due to the through-hole, a micro-short circuit may occurbetween the positive electrode 18 and the negative electrode 19.

Thus, the ratio of the mass of the base material layer per unit area tothe spatial volume of the negative composite layer is specified to be0.26 or more. As a result, even in the case where the base materiallayer melts, the formation of a through-hole in the base material layercan be suppressed. As a result, a micro-short circuit in the energystorage device at the time of heat generation can be suppressed.

It is preferable that the ratio of the mass of the base material layerper unit area to the spatial volume of the negative composite layer is0.31 or more. As a result, a micro-short circuit in the energy storagedevice can be even more suppressed.

In addition, it is preferable that the ratio of the mass of the basematerial layer per unit area to the spatial volume of the negativecomposite layer is 0.53 or less. As a result, while suppressing amicro-short circuit, the capacity of the energy storage device can beimproved.

(Density of Negative Composite Layer)

The density of a negative composite layer means the value obtained bydividing the mass of the negative composite layer by the apparent volumeof the negative composite layer. An apparent volume means the volumeincluding the void part. In the case where the negative composite layeris in a sheet shape, the apparent volume can be determined as theproduct of the thickness and the area of the negative composite layer.

Incidentally, the lower limit of the density of the negative compositelayer is preferably 1.3 g/cm³, and may be 1.45 g/cm³ or 1.5 g/cm³. Whenthe density is not less than the lower limit, the molten base materiallayer is more unlikely to penetrate into the negative composite layer.As a result, a micro-short circuit in the energy storage device can beeven more suppressed.

Meanwhile, the upper limit of the density of the negative compositelayer is 2.0 g/cm³, for example, and may be 1.8 g/cm³, 1.7 g/cm³, or 1.6g/cm³. When the density is not more than the upper limit, excellent iondiffusibility can be ensured, making it possible to possess sufficientdischarge capacity, for example. Then, it is preferable that the densityof a surface part of the negative composite layer which opposes to thebase material layer is larger than the density of a part of the negativecomposite layer adjacent to the negative electrode current collectionfoil. According to the configuration described above, the molten basematerial layer is more unlikely to penetrate into the negative compositelayer. As a result, a micro-short circuit in the energy storage devicecan be even more suppressed. Incidentally, the description that “thedensity of a surface part of the negative composite layer which opposesto the base material layer is larger than the density of a part of thenegative composite layer adjacent to the negative electrode currentcollection foil” specifically means that “in the case where the negativecomposite layer is bisected in the thickness direction, the density ofthe surface part of the negative composite layer which opposes to thebase material layer is larger as compared with the density of the otherpart of the negative composite layer adjacent to the negative electrodecurrent collection foil”.

(Electrolyte Solution)

As the electrolyte, an electrolyte solution obtained by dissolving anelectrolyte salt in a nonaqueous solvent may be used. In the case 11,the positive composite layer, the negative composite layer, and theseparator 21 are impregnated with the electrolyte solution. Theelectrolyte is not limited, and electrolytes generally proposed for usein nonaqueous electrolyte secondary batteries, such as lithium-ionsecondary batteries, may be used. Examples of solvents include cycliccarbonates such as propylene carbonate, ethylene carbonate, butylenecarbonate, chloroethylene carbonate, and vinylene carbonate; cyclicesters such as γ-butyrolactone and γ-valerolactone; linear carbonatessuch as dimethyl carbonate, diethyl carbonate, and ethylmethylcarbonate; linear esters such as methyl formate, methyl acetate, andmethyl butyrate; tetrahydrofuran and derivatives thereof; ethers such as1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, andmethyl diglyme; nitriles such as acetonitrile and benzonitrile;dioxolane and derivatives thereof and ethylene sulfide, sulfolane,sultone, and derivatives thereof. They may be used alone, and it is alsopossible to use a mixture of two or more kinds, for example.Incidentally, known additives may also be added to the electrolyte.

As the electrolyte salt, for example, inorganic ionic salts containingone of lithium (Li), sodium (Na), and potassium (K) such as LiClO₄,LiBF₄, LiAsF₆, LiPF₆, LiSCN, LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, NaClO₄, NaI,NaSCN, NaBr, KClO₄, and KSCN; and organic ionic salts such as LiCF₃SO₃,LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂), LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, (CH₃)₄NBF₄, (CH₃)₄NBr, (C₂H₅)₄NClO₄, (C₂H₅)₄NI,(C₃H₇)₄NBr, (n-C₄H₉)₄NClO₄, (n-C₄H₉)₄NI, (C₂H₅)₄N-maleate,(C₂H₅)₄N-benzoate, (C₂H₅)₄N-phthalate, lithium stearylsulfonate, lithiumoctylsulfonate, and lithium dodecylbenzenesulfonate, can be used. Theseionic compounds may be used singly or in combination of two or morethereof.

It is preferable to use LiBF₄ and a lithium salt includingperfluoroalkyl group such as LiN(C₂F₅SO₂)₂ as a mixture because aviscosity of the electrolyte can be decreased, thereby a low temperaturecharacteristic can be enhanced and a self-discharge can be suppressed.

In addition, it is also possible to use an ambient temperature moltensalt or an ionic liquid as the electrolyte.

(Other Constituent Members)

As other constituent members of the energy storage device, a terminaland the like can be mentioned. In the energy storage device of thepresent invention, as these constituent members, those conventionallyused may be suitably employed.

(Power Storage Apparatus 40)

One or more of the energy storage devices of the present invention maybe used to constitute a power storage apparatus. FIG. 3 shows oneembodiment of the energy storage apparatus. The power storage apparatus40 includes a plurality of power storage units 30. Each energy storageunit 30 includes a plurality of nonaqueous electrolyte secondarybatteries 10. The power storage apparatus 40 can be mounted, as shown inFIG. 4, as a power source for an automotive 100, such as an electricvehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybridelectric vehicle (PHEV).

Examples

Hereinafter, the present invention will be described in detail based onexamples and comparative examples. The present invention is not limitedto the following examples by any means. Incidentally, in the presentinvention, as described later, the resistance between a positiveelectrode and a negative electrode was measured to examine the effectsof the present invention.

(Positive Electrode)

The positive electrode was prepared as follows. 90 parts by mass of alithium composite oxide represented by compositional formulaLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ as a positive active material, 5 parts bymass of polyvinylidene fluoride as a binder, and 5 parts by mass ofacetylene black as an electrically conductive auxiliary were mixed.N-methylpyrrolidone (NMP) was suitably added thereto to form the mixtureinto a paste, thereby preparing a positive composite. The positivecomposite was applied to both sides of a positive electrode currentcollection foil formed of an aluminum foil having a thickness of 15 μmand dried, thereby forming a positive composite layer. Subsequently,pressure was applied using a roll press, thereby preparing a positiveelectrode. On the positive electrode, a region where the positivecomposite layer was not formed, and the positive electrode currentcollection foil was exposed, was provided (positive compositelayer-absent region), and a positive electrode lead was connected to theregion where the positive electrode current collection foil was exposed.

(Negative Electrode)

The negative electrode was prepared as follows. 95 parts by mass ofgraphite as a negative active material, 3 parts by mass ofstyrene-butadiene rubber (SBR) as a binder, and 2 parts by mass ofcarboxymethylcellulose (CMC) were mixed. Water was suitably addedthereto to form the mixture into a paste, thereby preparing a negativecomposite. The negative composite was applied to both sides of anegative electrode current collection foil formed of a copper foilhaving a thickness of 10 μm and dried, thereby forming a negativecomposite layer. Subsequently, pressure was applied using a roll press,thereby preparing a negative electrode. On the negative electrode, aregion where the negative composite layer was not formed, and thenegative electrode current collection foil was exposed, was provided(negative composite layer-absent region), and a negative electrode leadwas connected to the region where the negative electrode currentcollection foil was exposed.

The conditions for pressure application to the negative electrode andthe coating weight of the negative electrode mixture were changed,thereby preparing, as shown in the following Table 1, several negativeelectrodes having different porosities of the negative composite layerand different spatial volumes of the negative composite layer.Incidentally, in Table 1, the porosity of the negative composite layeris indicated as “negative electrode porosity (%)”, the coating weight ofthe negative electrode mixture as “negative electrode coating weight(g/100 cm²)”, and the spatial volume of the negative composite layer as“negative electrode spatial volume (cm³/100 cm²)”.

(Separator)

As the separator, a separator having an inorganic layer formed on thesurface of a polyolefin microporous membrane to serve as a base materiallayer or a separator composed only of a polyolefin microporous membraneto serve as a base material layer was used. The polyolefin microporousmembrane contains polyethylene and polypropylene as thermoplasticresins. The content of polyethylene was 95 mass %, and the content ofpolypropylene was 5 mass %. The thickness of the polyolefin microporousmembrane (base material layer) was 16 μm. In addition, with respect tothe mass of the polyolefin microporous membrane (base material layer)per unit area, as shown in Table 1, membranes having different masseswere used. The inorganic layer is formed by applying a composite forinorganic layer formation, which is obtained by mixing 95 mass % ofalumina particles (Al₂O₃) as an inorganic compound, 3 mass % ofpolyacrylic acid as a binder, and 2 mass % of carboxymethylcellulose asa thickener, and suitably adding water thereto to form the mixture intoa paste, to a surface of the base material layer, followed by drying.The thickness of the inorganic layer was 5 μm, and the particle size(D50) of the alumina particles was 0.7 μm.

Using the above constituent elements, the resistance was measured underthe conditions shown in Table 1.

(Measurement of Negative Electrode Porosity)

The porosity of the negative composite layer (negative electrodeporosity) was determined as follows. The thickness of the negativecomposite layer and the mass of the negative composite layer per unitarea were measured to calculate the negative composite layer density [A(g/cm³)], and, using the negative composite layer true density [B(g/cm³)] calculated from the true density of each material constitutingthe negative composite layer, the porosity was calculated from thefollowing equation. Incidentally, the thickness of the negativecomposite layer was measured using a micrometer.Negative electrode porosity (%)=[1−(A/B)]×100

Incidentally, the above negative electrode porosity may also becalculated as a value measured by “Mercury Intrusion Method” inaccordance with JIS-R1655 (2003), more specifically as the percentage ofvoids measured by the mercury intrusion method based on the followingrelational expression. D=−4σ cos θ/P (D: pore diameter, P: mercurypressure, σ: surface tension, θ: contact angle), provided that θ=130°and σ=484 mN/cm. Specifically, measurement is performed using a mercuryporosimeter (“WIN9400” manufactured by Micrometrics) with the pore sizemeasurement range being 0.005 to 20 μm.

(Measurement of Negative Electrode Spatial Volume)

The thickness of the negative composite layer was measured, and thevolume of the negative composite layer per unit area [C (cm³/100 cm²)]was calculated. The spatial volume of the negative composite layer(negative electrode spatial volume) was calculated from the followingequation using the volume of the negative composite layer per unit area[C (cm³/100 cm²)] and the porosity of the negative composite layer [D(%)]. Incidentally, the thickness of the negative composite layer wasmeasured using a micrometer.Negative electrode spatial volume(cm³/100 cm²)=(D/100)×C(Measurement of Mass of Base Material Layer Per Unit Area)

The base material layer was cut to a 10-cm square (10 cm×10 cm), and themass of the base material layer was measured. The value of the mass wasdefined as the mass of the base material layer per unit area.

(Measurement of Resistance)

Using the positive electrode, negative electrode, and separatordescribed above, the resistance was measured by the following method.

The positive electrode (30 mm×30 mm) and the negative electrode (32mm×32 mm) were stacked with the separator (40 mm×40 mm) therebetween.The separator was disposed between the positive electrode and thenegative electrode in such a manner that the inorganic layer of theseparator opposed to the positive electrode. The layered element of thepositive electrode, the separator, and the negative electrode wassandwiched between two plates made of SUS and pressed with a torque of0.5 N·m.

The layered element of the positive electrode, the separator, and thenegative electrode was stored in an oven set at 200° C. for 30 minutes,and the resulting resistance was measured. The resistance of the layeredelement was measured using a resistance meter RM3545 (manufactured byHioki E.E. Corporation) connected to the positive electrode lead and thenegative electrode lead.

By comparing the resistances measured as described above, whether amicro-short circuit has been suppressed can be judged. That is, when theresistance is large, it is estimated that the formation of athrough-hole in the base material layer has been suppressed, andconduction between the positive electrode and the negative electrode hasbeen inhibited. That is, this indicates the suppression of a micro-shortcircuit between the positive electrode and the negative electrode.

The results of the above measurements are shown in Table 1.

TABLE 1 Mass of base Negative electrode Separator material layer perNegative Negative Negative Negative Mass of base Presence/ Thicknessunit area/ electrode electrode composite electrode material layerabsence of of inorganic negative electrode porosity coating weight layerdensity spatial volume per unit area inorganic layer spatial volumeResistance (%) (g/100 cm²) (g/cm³) (cm³/100 cm²) (g/100 cm²) layer (μm)(g/cm³) (Ω/cm²) Example 1 26 1.01 1.594 0.165 0.085 Present 5 0.52 25.6Example 2 30 0.97 1.508 0.192 0.085 Present 5 0.44 20.2 Example 3 371.01 1.357 0.275 0.085 Present 5 0.31 22.7 Example 4 39 1.01 1.314 0.3000.085 Present 5 0.28 19.7 Example 5 41 1.01 1.271 0.326 0.085 Present 50.26 19.9 Comparative 49 1.01 1.099 0.450 0.085 Present 5 0.19 12.2Example 1 Comparative 49 1.18 1.099 0.526 0.085 Present 5 0.16 12.1Example 2 Example 6 30 0.97 1.508 0.192 0.060 Present 5 0.31 15.2Example 7 30 0.97 1.508 0.192 0.101 Present 5 0.53 40.3 Comparative 300.97 1.508 0.192 0.085 Absent 0 0.44 1.7 Example 3(Results and Discussion)

In Comparative Example 1, the negative electrode porosity is 49(%), andthe negative electrode spatial volume is as large as 0.450 (cm³/100cm²). Therefore, the value of the mass of the base material layer perunit area relative to the negative electrode spatial volume is as smallas 0.19 (g/cm³). In Comparative Example 1, the resistance was as smallas 12.2 (Ω/cm²). This is presumably because when the layered element ofthe positive electrode, the separator, and the negative electrode washeated, the base material layer melted and penetrated into the negativecomposite layer too much, resulting in a decrease in resistance. As aresult, in the energy storage device of Comparative Example 1, amicro-short circuit is likely to occur. In addition, in ComparativeExamples 2, similarly to Comparative Example 1, the negative electrodespatial volume is as large as 0.526 (cm³/100 cm²), and the value of themass of the base material layer per unit area relative to the negativeelectrode spatial volume is as small as 0.16 (g/cm³). As a result, alsoin the energy storage device of Comparative Example 2, a micro-shortcircuit is likely to occur.

In contrast, in Example 1 to Example 7, in which the value of the massof the base material layer per unit area relative to the negativeelectrode spatial volume is 0.26 (g/cm³) or more, the resistance is 15.2(Ω/cm²) or more, which is larger than in Comparative Example 1 andComparative Example 2. This is presumably because penetration of themolten base material layer into the negative composite layer wassuppressed. As a result, in the energy storage devices of Example 1 toExample 7, a micro-short circuit is suppressed.

In Comparative Example 3, the resistance is as small as 1.7 (Ω/cm²). Theseparator of Comparative Example 3 has no inorganic layer formed. As aresult, presumably, the separator underwent thermal shrinkage at thetime of heating and became unable to maintain its shape, resulting in adecrease in resistance. As a result, in the energy storage device ofComparative Example 3, a micro-short circuit is likely to occur.

In contrast, in Example 2, although the value of the mass of the basematerial layer per unit area relative to the negative electrode spatialvolume is the same as in Comparative Example 3, the resistance shown wasas high as 20.2 (Ω/cm²). This is presumably because the shape of theseparator was maintained by the inorganic layer, whereby the resistancewas kept high.

Example 3 and Example 6 will be compared. In Example 3, the value of themass of the base material layer per unit area relative to the negativeelectrode spatial volume is 0.31 (g/cm³), and also in Example 6, thevalue of the mass of the base material layer per unit area relative tothe negative electrode spatial volume is 0.31 (g/cm³).

Meanwhile, in Example 3, the mass of the base material layer per unitarea is 0.085 (g/100 cm²), and the resistance is 22.7 (Ω/cm²). InExample 6, the mass of the base material layer per unit area is 0.060(g/100 cm²), and the resistance is 15.2 (Ω/cm²). This is presumablybecause the mass of the base material layer per unit area of Example 3was 0.085 (g/100 cm²) or more, and thus the formation of a through-holein the base material layer was even more suppressed.

In Examples 1 to 3, the mass of the base material layer per unit area is0.085 (g/100 cm²), and the value of the mass of the base material layerper unit area relative to the negative electrode spatial volume is 0.31(g/cm³) or more. In Examples 1 to 3, the resistance is 20.2 (Ω/cm²) ormore, and thus a micro-short circuit is even more suppressed.

In Example 7, the mass of the base material layer per unit area is 0.101(g/100 cm²), and the value of the mass of the base material layer perunit area relative to the negative electrode spatial volume is 0.53(g/cm³), which are both large. In Example 7, the resistance is 40.3(Ω/cm²), and a micro-short circuit is even more suppressed.

In Examples 1 to 7, the content of polyethylene in the base materiallayer is 90 mass % or more. The melting point of polyethylene is loweras compared with polypropylene. Therefore, in the case where 90 mass %or more of polyethylene is contained in the base material layer, theapplication of the technology described herein is particularlyeffective.

As described above, according to the technology disclosed herein, amicro-short circuit in an energy storage device at the time of heatgeneration can be suppressed.

Other Embodiments

The technology disclosed herein is not limited to the embodimentsexplained in the descriptions and drawings above, and the followingembodiments, for example, are also included in the scope of thetechnology disclosed herein.

(1) The energy storage device according to this embodiment is configuredto be prismatic, but is not limited thereto, and may also have acylindrical shape or a shape in which an energy storage element isenclosed in an enclosure body formed of a laminate film. As necessary,an arbitrary shape may be employed.

(2) The energy storage device according to this embodiment is asecondary battery (nonaqueous electrolyte secondary battery), but is notlimited thereto, and may also be a primary battery, a capacitor, or thelike. An arbitrary energy storage device may be employed.

(3) The electrolyte solution may also be in gel form.

INDUSTRIAL APPLICABILITY

The present invention relates to an energy storage device and is capableof suppressing a micro-short circuit in the energy storage device at thetime of heat generation, and thus can be effectively used for automotivepower sources for electric vehicles and the like, power sources forelectronic devices, power sources for electricity storage, and the like.

DESCRIPTION OF REFERENCE SIGNS

-   -   10: Energy storage device    -   11: Case    -   16: Positive electrode terminal    -   17: Negative electrode terminal    -   18: Positive electrode    -   19: Negative electrode    -   20: Energy storage element    -   21: Separator    -   30: Energy storage unit    -   40: Energy storage apparatus    -   50: Vehicle main body    -   100: Automobile

The invention claimed is:
 1. An energy storage device comprising: apositive electrode; a negative electrode containing a negative compositelayer; and a separator disposed between the positive electrode and thenegative electrode, wherein the separator contains a base material layercontaining a thermoplastic resin and an inorganic layer formed on thebase material layer, the inorganic layer opposes to the positiveelectrode, the base material layer opposes to the negative electrode, amass of the base material layer per unit area is 0.085 (g/100 cm²) ormore, and a porosity of the negative composite layer is 47% or less. 2.The energy storage device according to claim 1, wherein a density of asurface part of the negative composite layer which opposes to the basematerial layer is larger than a density of a part of the negativecomposite layer adjacent to a negative electrode current collectionfoil.
 3. The energy storage device according to claim 1, wherein thebase material layer contains polyethylene as the thermoplastic resin,and a content of polyethylene is 90 mass % or more relative to a mass ofthe base material layer.
 4. An energy storage device comprising: apositive electrode; a negative electrode containing a negative compositelayer; and a separator disposed between the positive electrode and thenegative electrode, wherein the separator contains a base material layercontaining a thermoplastic resin and an inorganic layer formed on thebase material layer, the inorganic layer opposes to the positiveelectrode, the base material layer opposes to the negative electrode,and a ratio of a mass of the base material layer per unit area to aporosity of the negative composite layer is 0.20 (g/100 cm²) or more. 5.The energy storage device according to claim 4, wherein a mass of thebase material layer per unit area is 0.085 (g/100 cm²) or more.